How to Design a Dynamic Flexible PCB
Dynamic flex PCBs are the go-to solution for solving a wide range of interconnecting challenges in applications that often require extensive motion. These circuits are often used for high-density interconnects while occupying very little space in devices.
The most important quality of dynamic flex PCBs is their ability to stay operational in highly repetitive environments. However, designers must configure the copper circuits so they don’t exceed their maximum ductility threshold. Exceeding the ductility threshold will eventually result in circuit deformities, such as the formation of cracks and the hardening of the copper itself.
These challenges require designers to take a slightly different approach to dynamic flex PCB design. Let’s take a look at important tips for designing dynamic flex PCBs.
Type of Copper Used and Grain Direction
Designers can choose from two types of copper for dynamic flex PCBs. These are as follows:
- Rolled Annealed (RA)
- Electro Deposited (ED)
Rolled annealed copper is the industry standard for flex circuit designs. It is incredibly robust and has found use in a number of applications. RA copper tends to be a bit more forgiving than ED. Thanks to the smooth surface of RA copper (compared to ED), it is utilized in high-speed electronic applications.
ED copper is often used for its superior surface adhesion and slightly improved conductivity. Designers often use ED copper for rigid PCBs.
Designers transform the grain structure of RA copper by subjecting it to the rolled annealed process. This changes its grain direction from the initial vertical grain structure (similar to ED copper) to the horizontal and elongated structure that makes it useful for dynamic flex PCBs.
The end result is that the process improves the ductility of the copper and minimizes the risk of forming micro tears in the vertical axis. Furthermore, the process allows the copper to have a grain direction because of the elongation of the grain structure.
RA copper (with the grain direction along the length of the flex PCB) is used in all dynamic flex applications. Designers can choose RA copper in a range of thicknesses including 1 ounce, 2 ounces, and even ½ ounce.
The Minimum Bend Radius
Industry standards dictate at least 100X the finished thickness of flex circuits in the dynamic flex area of the circuit. This is absolutely essential to the functionality of the dynamic flex circuit. It is not uncommon for PCB designs to utilize both dynamic flex and static areas.
This can further complicate the design rules for dynamic flex PCBs because the rules will differ for the two types of areas. For example, flex circuits with an approximate thickness of 0.0005” will require a minimum bend radius of 0.500” or a minimum bend diameter of about 1.000” to ensure the longevity of the circuit.
Designers can adjust the minimum bend radius based on the thickness of the materials used in circuit design. Designers can reduce the minimum bend radius capability by utilizing thinner copper with a thin circuit core and coverlay layers. However, this can add to the costs of the circuit.
A big problem in dynamic flex PCBs is work hardening during repeated bending. Copper tends to harden due to repeated cycling and becomes prone to tears. Designers should allow for a larger bend radius to improve the circuit’s durability.
When the flex PCB bends, its neutral bending axis will shift toward the bend’s interior. This is important in dynamic flex PCBs because it restricts the number of allowed copper layers to a low number, usually a single layer.
Dynamic flex circuit designs are often used for applications such as rotating actuators, rotating assemblies, print head interconnects, and moving display interconnects.
Gerber Layer Count
It’s true that copper is very ductile, but it is prone to hardening when subjected to increased stress. This is why it is recommended to have a 1-layer construction - or keeping the layer count to a minimum.
This allows the copper circuits to coincide with the neutral bend axis (also known as the center of the construction), which is an optimum arrangement because it subjects the copper to the least amount of tensile and compressive forces.
Designers may use a 2-layer construction as long as they use a thin adhesiveless flex core (no thicker than 0.001”) between the two layers. This will minimize the distance of the circuits from the neutral bend axis and minimize the tensile forces during bending.
Three layer counts and above are not recommended for dynamic flex PCBs. If you must use three layers, make sure that the minimum bend radius is higher than 100X.
Principles of Shielding for Dynamic Flex PCBs
Designers are recommended to use shielding films instead of relying on copper or silver ink layers for EMI and RF purposes. This is because copper and silver ink layers contribute to the thickness of the design and limit the circuit's bending capabilities. Shielding films happen to be highly flexible, incredibly thin, and can be attached to one or both sides of a flex circuit.
An electrically conductive adhesive is used to interconnect shielding films to a ground circuit using selective openings in the coverlay. Flex circuits with a 2 layer design can use non-plated holes to allow the two shield layers to interconnect.
In all cases of dynamic flex PCB design, a thicker overall flexible stackup will require a larger bending radius. This keeps the amount of compressive and tensile stress to a minimum while forming the circuit to the desired angle.
A final recommendation is to avoid cross hatched fills and to stagger traces from layer to layer to minimize the effect of I-beams.
Note that some designs may have complex requirements that can impact the board’s functionalities. For these designs, we recommended a consultation with an engineering team to ensure that your dynamic flex circuit is manufacturable and meets your requirements.
If you are looking to learn more about dynamic flex PCB designs and need a solution, talk to the experts at Hemeixin PCB here.